DESCRIPTION (provided by investigator): Our long-term goal is to understand the role and regulation of Mediator in the control of gene expression and cell fate specification within the vertebrate central nervous system. Mediator is a multiprotein interface between gene-specific transcription factors and the eukaryotic RNA polymerase II general transcription machinery. In this capacity, Mediator serves to channel regulatory signals from activator and repressor proteins to affect changes in gene expression programs that control diverse physiological processes, including cell growth and homeostasis, development, and differentiation. MED12, an Xq13-encoded 230 kDa Mediator subunit, plays an essential role in neuronal development through selective regulation of neuronal gene expression. Genetic variation in MED12 has been linked to neuropsychiatric illness and X-linked mental retardation (XLMR). However, the molecular bases by which MED12 controls neuronal differentiation through selective gene regulation and the means by which pathologic sequence alterations impact MED12 function leading to behavioral and cognitive defects remain to be clarified. In this regard, we recently identified a functional interaction between the MED12 interface in Mediator and G9a histone methyltransferase required for epigenetic silencing imposed by the RE1 silencing transcription factor/neuron restrictive silencing factor (REST/NRSF), a key determinant of neuronal fate that suppresses neuronal gene expression in non-neuronal and neural progenitor cells. Notably, we found that missense mutations in MED12 responsible for two XLMR disorders, FG syndrome and Lujan syndrome, disrupt its REST-specific corepressor function, thus linking REST-dependent neuronal gene repression with higher-order cognitive function in humans. Because our recent studies implicate REST-dependent neuronal gene repression in epigenetic restriction of neural progenitor cell differentiation, our findings provide a possible epigenetic perspective to explain the role of MED12 in the etiology of XLMR through altered neuronal development. Thus, we hypothesize that XLMR- associated mutations in MED12 disrupt REST-imposed epigenetic restrictions on neuronal gene expression and neural progenitor cell differentiation. To provide support for this hypothesis, we propose the following aims to decipher the role and pathologic implications of MED12/Mediator in REST-dependent epigenetic suppression of neuronal gene expression and differentiation: (1) Elucidate the mechanistic basis of MED12/Mediator in REST-dependent extra-neuronal gene silencing; (2) Elucidate the role and regulation of MED12/Mediator in REST-dependent suppression of neuronal gene expression and differentiation in neural progenitor cells; (3) Elucidate the impact and mechanism of XLMR-associated mutations in MED12 on REST- dependent suppression of neuronal gene expression and differentiation in neural progenitor cells. We expect these studies to have important human health implications for cell replacement therapy in neurological disease as well as the etiology of XLMR.